Pentatricopeptide repeat proteins

Mitochondria and chloroplasts host genomes inherited from their bacterial ancestors. Their biogenesis thus requires fully functional gene expression machineries. RNA metabolism in organ-elles is particular and has attracted considerable attention since it combines prokaryotic features with unique traits that evolved in their eukaryotic host cell. For a long time, most of the molecular factors governing these processes have remained elusive. Recent research has shown that proteins of the pentatricopeptide repeat (PPR) family are major players of organelle gene expression. PPR proteins make a large group of eukaryote-specific RNA-binding proteins encoded in the nucleus and predominantly located to organelles. This protein family has been identified over a decade ago and has been the subject of a fast growing number of studies ever since. PPR proteins have escaped identification until complete genomic sequences were available because their primary sequences are very degenerate. Their identifying features are motifs of ca. 35 amino acids occurring as tandem arrays. Contrary to the sequence, the tri-dimensional organization of these modules seems highly conserved, with each repeat arranged in a helix-turn-helix structure and the succession of PPR motifs forming a solenoid. 5 PPR proteins are ubiquitous in eukaryotes and are totally absent from bacteria, with the exception of a small number of plant parasites such as Ralstonia that probably acquired PPR genes by horizontal gene transfer. 3 Still, the distribution of PPR genes among eukaryote clades is very uneven. They are for example present in relatively small numbers in fungi and animals, they occur in higher numbers in trypanosomes, and are present in very large numbers in higher plants. In the latter, hundreds of PPR genes are present, representing as much as 1% of nuclear genomes. 2 The identification and functional analysis of PPR proteins has already helped to solve long-standing questions regarding organ-elle gene expression. It has for example enabled to understand how sequence specificity is achieved for hundreds of cytidine to uridine editing sites present in higher plant organelle transcrip-tomes. 6 It has also settled the long-lasting debate on the existence of protein-only enzymes holding RNase P activity. Since their discovery, the function of PPR proteins has been associated with most aspects of gene expression in organelle, i.e. translation. Research on PPR proteins is thus instrumental to understand organelle RNA metabolism. Initial functional data on PPR proteins have mainly been derived from genetic studies. The most recent advances now unravel the mode of action of …